The invention concerns an automatic clutch in the drive train of a motor vehicle, having a motorized adjusting unit which actuates the clutch and is controlled by a system of sensors which allows a moment respectively transmitted by the clutch to be determined. The system of sensors senses whether the clutch is operating with slip and assigns the value of the transmitted moment of the clutch when slip is occurring to the respective value of the travel of the adjusting unit as the moment which can be transmitted by the clutch. The system further evaluates the moment value together with the travel value for adapting a characteristic curve or table of values reflecting the dependence between the clutch moment and the travel of the adjusting unit.
In motor vehicles having customary internal combustion engines, a transmission is arranged in the drive train. The transmission allows the transmission ratio between the speed of the vehicle engine and the speed of the drive wheels to be changed according to the respective traveling speed and loading of the vehicle. In the case of customary manually shifted transmissions, during a change of drive position the power flow between the engine and the drive wheels has to be interrupted by releasing a clutch.
It is known in principle to use automatic clutches for this purpose, i.e. the clutch is automatically released when changing the transmission stage and is subsequently re-engaged.
In addition, the clutch is also automatically controlled when starting the vehicle. As soon as a drive position suitable for starting has been selected, the clutch can, for example, be automatically engaged lightly in such a way that a small torque adequate for making the vehicle start to creep forward is transmitted. As soon as the engine power and the engine speed are increased by actuating a gas pedal or some other power control element, the clutch is automatically controlled in the sense of increasing the transmissible torque. To put it in simple terms, the clutch is then increasingly engaged.
These known automatic clutches operate in the manner of friction clutches, the frictional engagement of which is controlled by the assigned adjusting unit of the clutch.
The friction linings of such clutches are subjected to unavoidable wear. This has the consequence that the maximum torque which can be transmitted by the clutch may fall below the torque which can be generated by the engine and the clutch may, as a result, "slip" comparatively frequently. The frictional heat thereby produced may lead to the clutch being destroyed.
In the case of a manually actuated clutch, the state of wear can often be detected from the amount of pedal travel or the actuating force, or in some other way from the "actuating feel".
In the case of an automatic clutch, it is known in principle to monitor parameters correlated with the clutch wear.
In the case of an automatic clutch known from German Patent document DE 30 43 348 A1, the point of engagement of the clutch is detected by sensing the operating behavior of an electric servo motor of the clutch. A characteristic change in the electrical drive current of the electric motor occurs at the point of engagement. The actual position of the point of engagement of the clutch determined in this way is reported to a set point generator. The set point generator is provided as part of the clutch control and predetermines parameter-dependently the desired positions of the electric motor in the clutch control. Consequently, the setpoint generator can adapt the emitted setpoint signals to the actual state. As soon as an excessive shift in the point of engagement is established, a wear signal is generated.
For monitoring the frictional loss of a friction clutch and for monitoring the wear of the friction lining, it is known from German Patent document DE 36 01 708 A1 to calculate the instantaneous frictional loss from the moment transmitted at a given instant by the clutch and the difference between the speed of the clutch on the input side and the speed of the clutch on the output side. The average frictional loss of the clutching operation is determined from this. Each time after starting to drive the vehicle, the average frictional loss values of the clutching operations occurring after starting are summed and, after taking into account standard values for cooling, are compared with a maximum permissible frictional loss value predetermined for the clutch. If a maximum permissible frictional loss value is exceeded, a warning signal for thermal overloading of the clutch is emitted. By monitoring the slip of the clutch in the state of thermal overloading, a criterion for impermissibly high wear of the friction linings of the clutch can be obtained.
It is known from French Patent document FR 2 649 366 A in the case of an automatic clutch to compare an actual characteristic curve between the clutch moment and the travel of the associated adjusting unit with stored limiting characteristic curves for a clutch having no wear or an inadmissibly worn clutch. This is done in order to be able to emit a warning signal in good time when the clutch is considerably worn.
German Patent document DE 43 24 810 A1 concerns an automatic clutch of the above-mentioned type and describes a control process for an infinitely variable flexible drive mechanism with a clutch serving as a limiter for the transmissible torque. In this case, the slipping moment of the clutch is determined cyclically or anticyclically and the level of the clutch pressure is compared with a characteristic curve. The comparison fixes a correction value by which the characteristic curve is adapted.
The object of the invention is thus to provide an automatic clutch with exact wear indication.
This object is achieved according to the invention by bringing the adjusting unit of the clutch at least for a short time into its end position for the engaged clutch under normal driving conditions in order to determine a remaining amount of travel. A warning signal is generated if the maximum transmissible clutch moment which can be achieved when the travel is exhausted falls below a threshold value.
The invention is based on the general idea of regularly sensing the moment actually transmitted by the clutch as well as the travel of the adjusting unit, and determining from this the maximum value for the torque which can be transmitted by the clutch when the travel available for the adjusting unit is exhausted. Alternatively, it is determined whether a predetermined threshold value of the transmissible moment is exceeded when the travel is exhausted. This makes use of the fact that the moment M.sub.K respectively transmitted by the clutch can be easily determined as follows from the respective engine torque M.sub.M and the moment of inertia of the engine J.sub.M, predetermined by the design of the engine, and the change in speed over time of the engine dw/dt: EQU M.sub.K =M.sub.M -J.sub.M dw/dt.
Since modern motor vehicle engines are provided with automatic engine control, which constantly senses the torque generated by the engine, or analogous variables, as well as the engine speed, the engine parameters necessary for the determination of the moment actually transmitted by the clutch are constantly available. Thus, if the travel of the adjusting unit is also sensed, it can be constantly established which torque is transmitted, or can be transmitted, in the respective position. This offers the possibility of generating a warning signal as soon as the "travel reserve" is no longer adequate.
According to the invention, it is constantly checked whether the clutch is slipping or not. For this purpose, the speed on the input side of the clutch (this speed is predetermined by the engine speed) may be compared with the speed on the output side of the clutch (this speed can be determined from the speeds of the drive wheels and the transmission ratio of the respective transmission stage). As soon as slippage of the clutch is established, the moment respectively transmitted by the clutch is assigned to the respective travel of the adjusting unit as the transmissible moment. The values mutually assigned are stored and constantly adapted, so that there is a characteristic curve or a corresponding table of characteristic values reproducing the moment which can be transmitted by the clutch as a function of the travel of the adjusting unit.
Otherwise, with regard to preferred features of the invention, reference is made to the claims and the following explanation of the drawing, on the basis of which particularly advantageous embodiments are described.